1. Field of the Invention
The present invention relates in general to imaging, and more particularly to an imaging method and apparatus employing Radial Reflection Diffraction Tomography.
2. Description of Related Art
Intravascular ultrasound (IVUS) imaging provides a method for imaging the interior of blood vessel walls. In standard acoustical techniques, a catheter with a rotating ultrasound transducer is inserted into a blood vessel. The transducer launches a pulse and collects the reflected signals from the surrounding tissue. Conventional ultrasonic imaging systems use B-mode tomography or B-scans, wherein images are formed from the envelope of the received display echoes returning to an ultrasonic transducer as brightness levels proportional to the echo amplitude and by assuming straight ray theory (i.e., geometrical optics). The brightness levels can then be used to create cross-sectional images of the object in the plane perpendicular to the transducer image. However, such images typically suffer from the consequences of ray theory of sound propagation, which does not model its wave nature.
A circumferential scan can be made by either rotating a single transducer (mechanical beam steering) or by phasing an array of transducers around a circumference (electronic beam forming). Typically, one ultrasound pulse is transmitted and all echoes from the surface to the deepest range are recorded before the ultrasound beam moves on to the next scan line position where pulse transmission and echo recording are repeated. When utilizing B-scan, the vertical position, which provides depth of each bright dot is determined by the time delay from pulse transmission to return of the corresponding echo, and the horizontal position by the location of the receiving transducer element.
Although B-scan IVUS images can be utilized to detect plaque and characterize its volume, the classification of plaque types by ultrasound is very difficult. Conventional B-scan images utilizes scattering, which, in turn, depends on the acoustic impedance dissimilarity of tissue types and structures. Although hard calcifications in some plaque can be detected using such a mismatch, the similarity in the acoustic properties of fibrous plaque and lipid pools prevents direct identification. Consequently, the size of the fibrous cap cannot be accurately estimated.
Diffraction tomography has additionally been applied to medical imaging problems for a number of years, usually in a standard circumferential through transmission mode. Furthermore, improved vascular images have been provided by utilizing time domain diffraction tomography, a technique capable of accounting for the wave propagation of the transmitted acoustic waves in addition to redundant information from multiple angular views of the objects imaged. A related B-mode approach that incorporates spatial compounding has also been employed to provide improved vascular images through multiple look angles.
Background information on rotational IVUS systems are described, for example, in U.S. Pat. No. 6,221,015 to Yock. Background information on phased-array IVUS systems are described, for example, in U.S. Pat. No. 6,283,920 to Eberle et al., as well as U.S. Pat. No. 6,283,921 to Nix et al. Multi-functional devices have been proposed in other areas of vascular intervention. For example, U.S. Pat. No. 5,906,580 to Kline-Schoder et al., describes an ultrasound transducer array that may transmit signals at multiple frequencies and may be used for both ultrasound imaging and ultrasound therapy. Therapeutic ultrasound catheters, are described, for example, in U.S. Pat. No. 5,725,494 to Brisken et al. and U.S. Pat. No. 5,581,144 to Cori et al., which describes another ultrasound transducer array that is capable of operating at multiple frequencies. However, none of the above devices and associated techniques from the above cited patents, are suited for rapid identification of objects, such as, but not limited to, vulnerable plaque or objects recessed in a bore hole, in accordance with the principles of the present invention.